Use of correlation combination to achieve channel detection

ABSTRACT

Combinations of correlation results are used to achieve detection of multiple coded signals at a receiver in a wireless communications system. The code applied to signals includes a lower rate code and a higher rate code. The lower rate code is a nested or tiered code such that it comprises at least two code sequences of the higher rate code. The received coded signal is correlated with the higher rate code using a single higher rate correlator to provide a higher rate code correlation result. The higher rate code correlation results are fed to two or more lower rate code correlators that combine multiple higher rate code correlation results, each using a different lower rate code, to provide corresponding lower rate code correlation results. The presence of at least one coded signal or mutually exclusive coded signals can be determined from the lower rate code correlation results.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/282,936, filed on Apr. 10, 2001. This application is also acontinuation-in-part of U.S. patent application Ser. No. 09/775,305filed Feb. 1, 2001 entitled “Maintenance Link Using Active/StandbyRequest Channels.” It is also related to U.S. patent application Ser.No. 09/738,934 filed Dec. 15, 2000 entitled “Reverse Link CorrelationFilter In Wireless Communications Systems.” The entire teachings of theabove applications are incorporated herein by reference.

BACKGROUND

Code Division Multiple Access (CDMA) modulation is a multi-user accesstransmission scheme in which signals from different users overlap bothin frequency and in time. This is in contrast with Frequency DivisionMultiple Access (FDMA) in which user signals overlap in time, but areassigned unique frequencies, and Time Division Multiple Access (TDMA) inwhich user signals overlap in frequency, but are assigned unique timeslots. CDMA signaling is frequently used in cellular communicationsystems between a base station within a cell and a plurality of accessunits, e.g., wireless handsets, in the possession of users within thecell. The CDMA transmitted signal for each user that broadcasts from theuser's access unit is spread over a wide bandwidth, which is greaterthan the initial user information bandwidth. Each user's signal isspread by a different spreading code to create a wideband spread. All ofthe spread wideband signals transmitted by the different users arereceived at the base station and form a composite received signal. Thereceiver at the base station distinguishes different users by using alocal copy (or local reference) of the spreading code, which isavailable to both the access units and the base station in the CDMAsystem. Such a process is called channelization.

In an exemplary CDMA system according to the IS-95 standard, channelsare defined for a reverse link, i.e., when an access unit istransmitting to a base station in the system, using a code called apseudorandom noise (PN) code. The receiver at the base station detectsthe desired signal from a particular user out of the composite signal bycorrelating the composite signal with the original PN code. All othersignals having codes that do not match the code for the desired usercode are rejected by the correlator.

An exemplary CDMA reverse link includes a plurality of channels, e.g.,access and traffic channels (or even more channel types depending on thedesign of the CDMA system). The traffic channel is used to transmit userdata and voice, as well as signaling messages. The access channel isused on the reverse link to communicate control information to the basestation. For example, when the access unit does not have a trafficchannel assigned, the access channel is used to make call originationsand to respond to pages and orders. The traffic channels are principallyused to communicate voice or data payload information but are also usedfor other functions.

SUMMARY

In presently proposed so-called third generation (3 G) systems, multipletraffic channels may be assigned to each user, and the traffic channelsmay be encoded at different rates. This requires a receiver to configurea correlator for different data rates such that a single output isproduced for a particular data rate. However, if multiple outputs andoptions are required, without a priori knowledge as to which channel isused, multiple codes must be searched, thus requiring multiplecorrelators. Such requirements contribute to the complexity and increasethe cost of the receiver design.

There is a need for a wireless system with a flexible, simple receiverdesign. A wireless communications system is particularly needed thatprovides a single correlator in the receiver which can be used toreceive multiple channels.

In general, the present invention relates to use of combinations ofcorrelation results to achieve detection of multiple coded signals at areceiver in a wireless communications system. One aspect of theinvention provides a method of detecting coded signals wherein the codeapplied to the signal includes a lower rate code and a higher rate code.The lower rate code is a nested or tiered code such that it comprises atleast two repetitions or two sequences of the higher rate code. Thereceived coded signal is correlated with the higher rate code using asingle higher rate correlator to provide a higher rate code correlationresult. The higher rate code correlation results are fed to two or morelower rate code correlators that combine multiple higher rate codecorrelation results, each using a different lower rate code, to providecorresponding lower rate code correlation results. The presence of atleast one coded signal can be determined from the lower rate codecorrelation results.

In an embodiment that uses a first lower rate code and a second lowerrate code, the presence of one or another of two mutually exclusivecoded signals can be determined from the corresponding first and secondlower rate code correlation results. In particular, the first and secondlower rate code correlation results are compared with each other todetermine the presence of either a first indication corresponding to thefirst lower rate code or a second indication corresponding to the secondlower rate code. In one embodiment, one of the two indicationscorresponds to a request by an access unit to enter an active mode inorder to communicate a data payload from the access unit to a basestation in a wireless communications system. The other indicationcorresponds to a notification by the access unit to the base stationthat the access unit desires to remain in a standby mode.

According to another aspect of the invention, N lower rate codes areused in the detection to provide M lower rate code correlation results.The presence of at least one coded signal can be determined from the Nlower rate code correlation results. The N lower rate codes can beselected from a set of M possible codes based on a priori systeminformation. The system information can be used to limit the hypothesisoutcomes, if any are known, such as the mutual exclusivity of thepresence of coded signals. In one embodiment, the set of M possiblecodes may represent data or instructions relating to a set of nearbybase stations that are candidates for possible cell handoff and N mayrepresent the subset of the M nearby base stations that are identifiedas actual active candidates based on system criteria such as signalstrength or signal-to-noise figure.

The lower rate codes are preferably orthogonal to each other and can beWalsh codes, Gutleber codes, maximum length (M)-sequences, orPN-sequences.

According to another aspect of the invention, detection of the receivedcoded signal is provided independent of the correlation method that isused. In particular, for a code applied to the signal that includes anested code, the nested code being one of a set of M possible nestedcodes, the detection method comprises correlating the received codedsignal to provide N nested code correlation results and determining thepresence of at least one coded signal from the N nested code correlationresults.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a general diagram illustrating a wireless communicationsystem.

FIG. 2 is a timing diagram illustrating heartbeat slot and link qualitymanagement (LQM) slot timing.

FIG. 3 is a diagram illustrating the relationship among tier 1, tier 2and tier 3 code sequences.

FIG. 4 is a diagram illustrating a selected set of codes in a tieredcode structure.

FIG. 5 is a block diagram of channel encoding at a transmitter in thesystem of FIG. 1.

FIG. 6 is a block diagram of channel correlation at a receiver accordingto the principles of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is a diagram of a wireless communications system 100 according tothe principles of the present invention. A base station 25 maintainswireless communication links with a plurality of access units 42A, 42B,42C (collectively, access units 42) as shown. Such wireless links areestablished based upon assignment of resources on a forward link 70 anda reverse link 65 between the base station 25 and access units 42. Eachlink 65 or 70 is typically made up of several logical channels 55 or 60.

The system 100 supports communications between interface 50 and network20. Network 20 is typically a Public Switched Telephone Network (PSTN)or computer network such as the Internet. Interface 50 is preferablycoupled to a digital processing device such as a portable computer (notshown), to provide wireless access to network 20.

In an illustrative embodiment, the forward link channels 60 and reverselink channels 55 are defined in the wireless communications system 100as Code Division Multiple Access (CDMA) channels. That is, each CDMAchannel is preferably defined by encoding data to be transmitted overthe channel with a channel code. The channel coded data is thenmodulated onto a radio frequency carrier. This enables a receiver todecipher one CDMA channel from another knowing only the particularchannel code assigned to that channel.

The forward link channels 60 include at least three logical channeltypes. Included among these are a Link Quality Management (LQM) channel60L, a paging channel 60P, and multiple traffic channels 60T.

The reverse link 65 includes heartbeat channels 55H, an access channel55A and multiple traffic channels 55T. Generally, the reverse linkchannels 55 are similar to the forward link channels 60 except that eachreverse link traffic channel 55T may support variable data rates from2.4 kbps to a maximum of 160 kbps.

Data transmitted between base station 25 and an access unit 42 typicallyconsists of encoded digital information, such as hypertext transferprotocol (HTTP) encoded Web page data. Based on the allocation oftraffic channels in the reverse link 65 or forward link 70, datatransfer rates are generally limited by the number of available trafficchannels 55T, 60T.

As shown in FIG. 2, the forward link LQM channel 60L is partitioned intoa predetermined number of periodically repeating time slots for thetransmission of messages to each of multiple access units 42. Eachaccess unit 42A identifies messages directed to itself based uponmessages received in an assigned time slot.

The reverse link heartbeat channel 55H is shared among multiple users.For example, the heartbeat channel 55H is also partitioned intoperiodically repeating time slots. Each time slot is assigned to one ofmany access units 42 for transmitting heartbeat messages to the basestation 25. Accordingly, the base station 25 identifies from whichaccess unit 42A a message is transmitted based upon the receipt of amessage in a particular time slot. The heartbeat channel 55H and the LQMchannel 60L are described in more detail below.

In the following description, reference is again generally made to FIG.1, but more specific details of LQM channel 60 and heartbeat channel 55Hare referenced to FIG. 2.

Generally, to establish a synchronized link with the base station 25,access units 42 transmit link request messages on the access channel 55Ato base station receiver 35 via access unit transmitter 40. Messages arethen acknowledged and processed at the base station 25. If available,resources are then allocated at the base station 25 to establish abidirectional communication link with the requesting access unit 42A.

Within the forward link 70, the paging channel 60P is used by the basestation transmitter 30 to send overhead and paging messages or commandsto the access unit receiver 45. Overhead information includes data suchas system configuration parameters for establishing wireless links withaccess units 42.

As mentioned previously, wireless communication system 100 includes aheartbeat channel 55H in the reverse link 65 and link quality managementchannel (LQM) 60L in the forward link 70. These channels are sharedbetween the base station 25 and multiple access units 42. That is, thebase station 25 transmits messages to multiple access units 42 using thesame forward link LQM channel 60L, where a message to a particularaccess unit 42A is transmitted in an assigned time slot. In this way,time slot assignments serve as a way of addressing messages to aparticular access unit and corresponding communication link.

The present system can support users that require on-demand, sporadichigh speed throughput. For example, remote users connected to theInternet over a wireless link typically require high speed throughputwhen downloading an object file such as a Web page. Such users thentypically do not require any data transfer for a period of time. Tosupport such users, it is advantageous to maintain synchronization withthe base station for future on-demand data transfers. This is achievedin the wireless communication system 100 by maintaining a minimalconnection with the base station 25 even when no data is being activelytransferred between the base station 25 and a specific access unit 42.

Repeatedly creating or reviving connections for users who sporadicallyneed a link can be time consuming and an inefficient use of resources.It is also inefficient to reserve resources such as traffic channels 55Tfor subscribers who are not transmitting data. Accordingly, trafficchannels 55T are allocated on an as-needed basis to support datatransfers, optimizing the use of available resources in wirelesscommunication system 100.

FIG. 2 is a timing diagram for the heartbeat channel 55H and LQM channel60L. Preferably, there are two LQM channels 60L and two heartbeatchannels 55H since channels are typically allocated in pairs. However,only one of each channel type is shown in FIG. 2 for illustrativepurposes.

As shown, 64 time slots (in each direction) are defined per EPOCH periodin each of the heartbeat 55H and LQM 60L channels. The EPOCH period inthe illustrated embodiment is 13.3 mS, so that each time slot is 208 mSor 256 code chips where a chip is a unit of time that corresponds to theoutput interval of the spreading code. Because time slots repeat on aperiodic basis, base station 25 exchanges information with a particularaccess unit 42A every EPOCH or 13.3 mS.

Data transmissions on the LQM channel 60L are maintained by the basestation 25, which is preferably used as a master timing reference. Thatis, timing of the access units 42 is aligned with base station 25.Access units 42, therefore, must synchronize themselves to the basestation 25, and specifically to the LQM channel 60L, in order tomaintain synchronization with the base station 25.

Generally, a link between the base station 25 and an access unit 42A ismaintained in one of three modes: active, standby or idle.Synchronization between base station 25 and a particular access unit 42Ais maintained only for the active and standby mode.

While in the active mode, synchronization of the forward and reverselink is maintained between the LQM channel 60L and traffic channels 55Tsince the heartbeat channel time slot is no longer dedicated on thereverse link 65 to the access unit 42A.

Each access unit 42A in the standby mode is assigned one time slot inthe forward link LQM channel 60L and one time slot in the reverse linkheartbeat channels 55H. Accordingly, information is targeted to areceiving access unit 42A (subscriber) based upon the transmission of amessage in a particular time slot. For example, an access unit 42Aassigned to time slot #1 decodes information received in time slot #1 onthe forward link LQM channel 60L, while data is transmitted back to thebase station 25 from access unit 42A in time slot #1 of the reverse linkheartbeat channel 55H. Both base station 25 and access unit 42A identifyto which link a message pertains based on receipt of a message in aparticular time slot. It should be noted that although the LQM channel60L is used as the time reference as described above, the principles ofthe present invention equally apply where the heartbeat channel 55H isalternatively used as a master timing reference rather than the LQMchannel 60L. In other words, base station 25 is optionally synchronizedwith respect to an access unit 42A.

In the standby mode, synchronization is maintained between the forwardlink LQM channel 60L and reverse link heartbeat channel 55H based uponmessages sent in the appropriate time slot on the LQM channel 60Lindicating to a particular access unit 42A whether messages transmittedto the base station 25 from that access unit are received in theappropriate time slot. Message transmissions from the access unittransmitter 40 to base station 25 on the heartbeat channel 55H areanalyzed at base station receiver 35 to achieve fine tuning alignmentbetween base station 25 and each of multiple access units 42.

As shown in FIG. 2, time slots A₁ through A₁₆ of the LQM channel 60L arereserved for access units 42 in the active mode, indicating that data isbeing transferred between the access unit 42A and the base station 25.Contrariwise, time slots numbered 1-48 are reserved for access units 42operating in the standby mode on the LQM channels 60L.

At any given time, there are typically no more than 48 time slots in theheartbeat channel 55H or LQM channel 60L assigned to respective accessunits 42. This ensures that on completion of a data transfer between anaccess unit 42A and base station 25, an access unit 42A in the activemode assigned an active time slot can revert back to the standby modeand consequently be assigned an unused standby mode time slot 1-48 inthe LQM channel 60L again.

The details relating to use of the LQM channel 60L and heartbeatchannels 55H for synchronization and timing alignment are disclosed inthe above-mentioned U.S. patent application Ser. No. 09/775,305.

A set of channel codes are used at the access units 42, one code ofwhich is generally to be transmitted in the assigned time slot in thereverse link heartbeat channel 55H. The transmission of this code isused as a signal received by the base station 25 to retainsynchronization with the access unit 42A while in a “standby” mode. Eachcode however, may also correspond to a particular command or request.For example, one code is used to notify the base station that the accessunit 42A is ready to begin transmitting a data payload to the basestation, i.e., an access unit requests to go into an “active”transmission mode. This is referred to herein as a “heartbeat withrequest” signal. Another code is used to notify the base station thatthe access unit desires to remain in standby mode. This is referred toherein as a “heartbeat” signal.

The wireless system according to the invention provides three tiers ofdata rates, i.e., tier 1, tier 2, and tier 3, for use by the CDMAchannels. At tier 1, a transmitter transmits 8 chips per symbol to areceiver. At tier 2, the transmitter transmits 32 chips per symbol tothe receiver. At tier 3, the transmitter transmits 128 chips per symbol.

FIG. 3 shows the relationship between tiers 1, 2 and 3 in more detail.In particular, what is shown is a nesting of the codes. A tier 1 codecomprises an 8 chip sequence J₁ through J₈. The tier 2 code comprises 4code elements, K₁ through K₄. Each of the code elements K₁ through K₄ iscomposed of, is aligned with, and has a duration equal to, a tier 1 codesequence J₁ through J₈. That is, the code boundary of the tier 1 codecoincides with each of the tier 2 code elements K₁ through K₄. Thus, thetier 2 code repeats every 32 chips. Likewise, the tier 3 code comprisescode elements, L₁ through L₄. Each code element of the tier 3 code iscomposed of, is aligned with a corresponding tier 2 code sequence, K₁through K₄. Thus, the tier 3 code sequence, L₁ through L₄, has aduration of 128 chips.

In the preferred embodiment, the difference between the channel codeassigned to the heartbeat signal versus the heartbeat with requestsignal is the specific tier 3 code that is applied. That is, the channelcodes assigned to the heartbeat and heartbeat with request signals areselected such that the tier 1 and tier 2 codes are the same for eachsignal. The difference is only in the tier 3 code sequence that neststhe tier 1 and tier 2 codes. The nesting of the tier 1 and tier 2 codeswith respect to the tier 3 codes for the heartbeat and heartbeat withrequest signals is illustrated in FIG. 4, which shows a tree structurefor the tiered codes. In particular, four codes that are assigned toheartbeat and heartbeat with request signals are indicated as individualbranches connected to a common tier 2 branch that is in turn connectedto a particular tier 1 branch. Other branches are shown to indicatechannel code assignments for other channels, e.g., traffic, maintenanceand access channels. The notation (X, Y, Z) is used to indicate thebranches assigned at each tier to the particular code. Thus, one code(7,3,0) is reserved for the heartbeat signal while another code (7,3,1)is reserved for the heartbeat with request signal. Another optionalheartbeat signaling pair uses codes (7,3,2) and (7,3,3). Note that otherchannels (e.g., traffic, access and maintenance) can be assigned otherunique codes, as shown in the tree structure.

The tier 3 codes are preferably orthogonal to each other. The orthogonalcodes can be Walsh codes or Gutleber codes or other code such as maximallength (M)-sequences or PN-sequences. It should be noted that while athree-level or three-tiered code is used in the preferred embodiment,other embodiments can use two tiers. For example, a 64 chip tier 1 codenested in a four element tier 2 code, that is, 256 chips in length couldbe used. Another two-tiered code includes a 16 chip tier 1 code nestedin an eight element tier 2 code, that is, 128 chips in length.

Turning attention now to FIG. 5, the channel encoding process fortransmission of heartbeat and heartbeat with request signals on theheartbeat channel 55H of the reverse link 65 from a transmitter 40 ataccess unit 42A is described. Specifically, the channel encoding processtakes an input data signal 101 that represents information to betransmitted. In the case of a heartbeat or heartbeat with requestsignal, the data has a value of 1 for the duration of the time slot,i.e., 256 code chips. A serial to parallel converter 102 provides anin-phase (i) and quadrature (q) signal path to a pair of multipliers106-i and 106-q. A spreading code generator 104 provides a spreadingcode used for spectrum spreading purposes. Typically, the spreading codeis a short pseudorandom noise code.

A second code modulation step is applied to the (i) and (q) signal pathsby multiplying the two signal paths with a tier 1 code. This isaccomplished by the tier 1 code generator 110 and code multipliers 120-1and 120-q.

A third step in the encoding process is to apply a tier 2 code asgenerated by tier 2 code generator 112. This is accomplished by themultipliers 122-i and 122-q impressing the tier 2 code on each of thein-phase and quadrature signal paths.

In a fourth and final step of the encoding process, a tier 3 code isapplied to the (i) and (q) signal paths. This is accomplished by thetier 3 code generator 114 and the code multipliers 124-i and 124-q. Asnoted previously, the tier 3 code (x) for sending the heartbeat signalis selected to be different from the tier 3 (y) code selected forsending the heartbeat with request signal.

The tier 3 encoded in-phase and quadrature signal paths modulate acarrier wave as generated by carrier wave source 126 using an RFmodulator 128. The modulated signal is amplified through amplifier 130and transmitted via antenna 132.

A chip clock 108 provides chip clock timing at the rate of 1.2288 MHz tothe tier 1, tier 2, and tier 3 generators 110, 112 and 114. As notedpreviously, the tier 1 code is at a rate of 8 chips per symbol. The chipclock is divided down by a factor of 8 using divider 116. The tier 2code generator operates at 32 chips per symbol. The chip clock isdivided again by a factor of 4 by divider 118 for the tier 3 generator114 which provides 128 chips per symbol.

FIG. 6 is a block diagram that illustrates channel correlation at areceiver 35 of base station 25 (FIG. 1) in accordance with principles ofthe present invention. In general, the correlation process takesadvantage of the structure of the tiered or nested codes used torepresent coded signals, for example the heartbeat and heartbeat withrequest signals in the present system. In particular, the correlationprocess uses the output of a higher rate correlator to feed two or morelower rate correlators, as described further below. Therefore, thehigher rate correlator structure can be shared to achieve detection ofmultiple coded signals.

The channel correlation process includes a number of codes as generatedby spreading code generator 220, tier 1 code generator 208 and tier 2code generator 210. In addition, to detect separate coded signals thatare coded at the tier 3 code rate, corresponding separate codes aregenerated by tier 3 code generators 212 x and 212 y, respectively.

As shown in FIG. 6, a signal 203 received by antenna 202 is fed into RFdemodulator 204 where the signal is demultiplexed to provide in-phase(i) and quadrature (q) signal paths to a first pair of multipliers 222-iand 222-q. Spreading code generator 220 provides a spreading code usedfor despreading purposes. This spreading code is the same as thespreading code used in the encoding process with spreading codegenerator 104 (FIG. 5).

A second step in the correlation process is to apply the tier 1 code asgenerated by tier 1 code generator 208. This is accomplished by themultipliers 224-i and 224-q impressing the tier 1 code on each of thein-phase and quadrature signal paths.

In a third step of the correlation process, the tier 2 code as generatedby the tier 2 code generator 210 is applied to each of the in-phase andquadrature signal paths by multipliers 226-i and 226-q.

In the final step of the correlation process, a particular tier 3 codeas generated by the respective tier 3 code generators 212 x and 212 y isapplied to each of the in-phase and quadrature signal paths.

As shown in FIG. 6, there are two correlation legs 227 x and 227 y thatshare the tier 2 correlation results. In the illustrated embodiment,where 2 possible codes could have been sent (x for heartbeat and y forheartbeat with request), there are two tier 3 correlators. The tier 3codes are applied by respective multipliers 228 x-i, 228 x-q and 228y-i, 228 y-q. Each correlation leg 227 x and 227 y includes integrators230 x-i, 230 x-q and 230 y-i, 230 y-q. In addition, in the in-phase andquadrature signal paths of each leg 227 x and 227 y are includedsquarers 232 x-i, 232 x-q and 232 y-i, 232 y-q. The outputs of the valuesquarers are summed in summers 234 x and 234 y respectively to providefinal correlation outputs 236 x and 236 y, respectively.

As configured, the integrators 230 integrate over 128 chips. In otherembodiments, the integration can be distributed at each tier stagerather than at the final tier 3 stage as shown in FIG. 6.

The correlation in FIG. 6 can be viewed as a series of correlations atthe succeeding tiered code rates. That is, the received coded signal iscorrelated with the higher rate code (tier 2) using a single higher ratecorrelator (tier 2 code generator 210, multipliers 226-i, 226-q) toprovide a higher rate code correlation result. In a sense, the higherrate code correlation result is a sub-correlation that corresponds to acode element of the lower rate code. The higher rate code correlationresults are then fed to two or more lower rate code correlators(correlation legs 227 x, 227 y) that combine multiple higher rate codecorrelation results, each using a different lower rate code (tier 3 codegenerators 212 x, 212 y), to provide corresponding lower rate codecorrelation results (236 x, 236 y). The presence of at least one codedsignal can be determined from the lower rate code correlation results.Thus, the dual outputs 236 x, 236 y are generated in part from the samesub-correlations or higher rate code correlation results.

In particular, the presence of one or another of two mutually exclusivecoded signals can be determined from the lower rate code correlationresults. For example, the lower rate code correlation results 236 x, 236y can be compared with each other to determine the presence of eitherheartbeat (code x was sent) or heartbeat with request (code y was sent)signals.

It should be understood that while two correlation legs 227 x and 227 yare shown in FIG. 6, it is possible to have multiple such correlationlegs to use combinations of correlation results to achieve detection ofmultiple coded signals. For example, there can be a set of M tier 3codes with a known subset of N selected codes to be used incommunicating coded signals. In that case, the correlator structure canbe expanded to have N different correlation legs 227, each one having adifferent tier 3 code generator 212 corresponding to the N selectedcodes.

Accordingly, the N lower rate codes can be used in the detection toprovide M lower rate code correlation results. That is, in general, thepresence of at least one coded signal can be determined from the N lowerrate code correlation results. The N lower rate codes can be selectedfrom a set of M possible codes based on a priori system information. Thesystem information can be used to limit the hypothesis outcomes, if anyare known, such as the mutual exclusivity of the presence of codedsignals.

In one embodiment, the set of M possible codes may represent a set ofnearby base stations that are candidates for possible cell handoff ofone or more of the code channels and N may represent the subset of the Mnearby base stations that are identified as actual active or preferredcandidates based on system criteria such as signal strength orsignal-to-noise figure.

For example, consider the process of hand over in a cellularcommunication system, where a mobile access unit is moving from an areaserviced by one cell site to another. To avoid disruption ofcommunications (e.g., dropping a call) while the access unit crosses acell boundary, the timing of handing over control to a new base stationmust be carefully orchestrated. In a process known as Mobile AssistedHand Over (MAHO) the mobile access unit performs certain calculations todetermine when to communicate to both the current serving base stationand a new serving base station that hand over is imminent. For CDMAbased systems that employ soft hand-off of the reverse link, this may betransmitted to both base stations simultaneously, but it is notrequired.

In this process, each access unit maintains a list of candidate basestations in its general vicinity. This can be done, for example, bydetecting the presence of forward link paging channels 60P or pilotchannels from various base stations 25 in the vicinity (FIG. 1). At anygiven time, this candidate list will consist of N of M possible basestations in the system 100. The access unit periodically sends thecandidate to each base station that it “sees,” such as on a reverse linktraffic channel 55T. However, precise timing of an actual need for handover (such as when the paging channel 60P or pilot channel from acurrently serving base station is diminishing in power) is critical.Accordingly, the access unit can use the invention by simply sending ashort burst with one of N possible tier 3 codes. Thus, because the basestation has the candidate list of N preferred base stations available,it can utilize N tier 3 correlators 227 with the N expected codes, anddetermine which one was sent. In this way, hand over control informationcan be rapidly and efficiently communicated for selecting service.

The embodiment illustrated in FIG. 6 shows sharing of the output of ahigher rate (i.e., tier 2) correlator with two or more lower rate (i.e.,tier 3) correlators. It should be understood, however, that in otherembodiments the output of the tier 1 correlator can be shared with twoor more tier 1 correlators that in turn are shared with two or more tier3 correlators depending on the types of nested codes used in thewireless communications system.

The correlation process described above with respect to FIG. 6 can betime multiplexed among different access units 42 (FIG. 1) that share theheartbeat channels 55H, thereby allowing a single correlator structureto be shared.

The tier 1, 2, and 3 codes are shown in FIGS. 5 and 6 as operationsusing Walsh codes. It should be understood that other orthogonal codessuch as Gutleber codes could be used as well as M-sequences or pseudoorthogonal codes.

It should also be understood that detection of the received coded signalcan be provided independent of the correlation method that is used wherethere is a priori system knowledge available. In particular, for a codeapplied to the signal that includes a nested code, the nested code beingone of a set of M possible nested codes, detection can be achieved bycorrelating the received coded signal to provide N nested codecorrelation results and determining the presence of at least one codedsignal from the N nested code correlation results using the systemknowledge to limit outcomes. The specific N nested codes can change overtime, with information indicating the changes in the current set ofcodes being communicated between base station and access units toprovide a priori system information that is current.

While the specific embodiments described herein relate to operation on areverse link, it should be understood that the principles of the presentinvention are also applicable to embodiments that detect coded signalson a forward link.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method of detecting a received coded signal in a receiver whereinthe code applied to the received coded signal includes a lower rate codecomprising at least two code sequences of a higher rate code, the methodcomprising: correlating the received coded signal at the receiver withthe higher rate code to provide a higher rate code correlation result;correlating the higher rate code correlation results with a first lowerrate code to provide a first lower rate code correlation result;correlating the higher rate code correlation results with a second lowerrate code to provide a second lower rate code correlation result; andcomparing the first lower rate code correlation result to the secondlower rate code correlation result to determine the presence of a signalindication.
 2. The method of claim 1, further comprising determining thepresence of a first indication corresponding to the first lower ratecode from the first lower rate code correlation result.
 3. The method ofclaim 1 further comprising determining the presence of a secondindication corresponding to the second lower rate code form the secondlower rate code correlation result.
 4. The method of claim 1 furthercomprising determining the presence of one or another of two mutuallyexclusive signal indications form the first and second lower rate codecorrelation results.
 5. The method of claim 1 wherein the received codedsignal is received periodically.
 6. The method of claim 1 wherein thelower rate codes are orthogonal to each other.
 7. The method of claim 1wherein the lower rate codes are Walsh codes.
 8. The method of claim 1wherein the lower rate codes are Gutleber codes.
 9. The method of claim1 wherein the lower rate codes are maximum length sequences.
 10. Themethod of claim 1 wherein the lower rate codes are PN-sequences.
 11. Themethod of claim 1 wherein the higher rate code is 32 chips in length andthe lower rate codes are 128 chips in length.
 12. The method of claim 1wherein the lower rate codes are used for timing signals.
 13. A methodof detecting a received coded signal in a receiver wherein a codeapplied to the received coded signal includes a lower rate codecomprising at least two code sequences of a higher rate code, the methodcomprising: demodulating the received coded signal at the receiver intoin-phase and quadrature components; correlating the in-phase andquadrature components with the higher rate code to provide higher ratecode correlation results; correlating higher rate code correlationresults with a first lower rate code to provide a first lower rate codecorrelation result; correlating higher rate code correlation resultswith a second lower rate code to provide a second lower rate codecorrelation result; and comparing the first lower rate correlationresult to the second lower rate code correlation result to determine thepresence of a signal indication.
 14. The method of claim 13 wherein thelower rate codes belong to a set of M possible lower rate codes, and thesteps of correlating the higher rate code correlation results are basedon N of the set of M lower rate codes to provide N lower rate codecorrelation results where N is less than M.
 15. The method of claim 14further comprising determining the presence of at least signalindication from the N lower rate code correlation results.
 16. Themethod of claim 14 further comprising determining the presence ofmutually exclusive signal indications from the N lower rate codecorrelation results.
 17. The method of claim 14 further comprisingdetermining the presence of N mutually exclusive signal indications fromthe N lower rate code correlation results.
 18. The method of claim 14further comprising determining the presence of a subset of N mutuallyexclusive signal indications from the N lower rate code correlationresults.
 19. The method of claim 14 wherein the N lower rate codes areselected based on a priori system information.
 20. The method of claim14 wherein the received coded signal is received periodically.
 21. Themethod of claim 14 wherein the lower rate codes are orthogonal to eachother.
 22. The method of claim 14 wherein the lower rate codes are Walshcodes.
 23. The method of claim 14 wherein the lower rate codes areGutleber codes.
 24. The method of claim 14 wherein the lower rate codesare maximum length sequences.
 25. The method of claim 14 wherein thelower rate codes are PN-sequences.
 26. The method as in claim 13 whereinthe lower codes are nested codes comprising at least two code sequencesof a base code and N nested lower rate codes are selected from of a setof M possible nested codes.
 27. The method of claim 26 furthercomprising determining the presence of mutually exclusive signalindications from the N lower rate code correlation results.
 28. Themethod of claim 26 further comprising determining the presence of Nmutually exclusive signal indications from the N lower rate codecorrelation results.
 29. The method of claim 26 further comprisingdetermining the presence of a subset of N mutually exclusive signalindications from the N lower rate code correlations results.
 30. Themethod of claim 26 wherein the N nested codes are selected based on apriori system information.
 31. The method of claim 26 wherein thereceived coded signal is received periodically.
 32. The method of claim26 wherein the nested codes are orthogonal to each other.
 33. The methodof claim 26 wherein the nested codes are Walsh codes.
 34. The method ofclaim 26 wherein the nested codes are Gutleber codes.
 35. The method ofclaim 26 wherein the nested codes are maximum length sequences.
 36. Themethod of claim 26 wherein the nested codes are PN-sequences.
 37. Themethod according to claim 13 further comprising using a higher rate codecorrelator with the higher rate code to correlate the in-phase andquadrature components with the higher rate code to provide higher ratecode correlator output; and sharing the higher rate code correlationoutput to feed two or more lower rate code correlators that each use adifferent one of the lower rate codes to provide the first and secondlower rate code correlation results.
 38. The method of claim 37 whereinN lower rate codes are selected from a set of M possible codes and the Nlower rate codes are selected based on a priori system information.